Most modern electric circuits are formed as "printed" circuits on a substrate, either through an additive process (e.g., plating or growing) or a subtractive process (e.g., etching). In the field of circuit fabrication, one of the more critical functions is the ability to control the size (length and width) of features, or critical dimensions (CDs). Any deviation in the dimensions of a feature can adversely impact on the performance of the resulting circuitry.
As metrology scanning electron microscopes (SEMS) are becoming more and more complicated, complex pattern recognition routines, measurement, and focusing algorithms are quickly making SEMs one of the most complicated tools on the production line. One problem associated with the use of SEMs that has been known for a long time is the need to obtain best focus and stigmation at a high degree of accuracy. Any improvement in focus and stigmation control techniques would enhance the quality of the CD SEM measurements.
Typically, conventional SEM tools have their focus checked manually. An operator looks at an image provided by the SEM. The operator adjusts the focus till he or she subjectively perceives that the tool is best focused. Conventional SEM process and tool control may not, however, be sensitive enough.
A second conventional focus method uses an optical microscope to map the "z" position of the features being measured optically, then uses a calibration equation, transposing the "z" position values to objective lens current values that correspond to a SEM focus value for each of the sites that was focused on optically. The assumption in this method is that the optical focusing technique is repeatable enough that errors are not introduced when the calibration process takes place. This method suffers from poor repeatability due to many variables that may contribute to poor calibration. Other optical techniques may also be used. For example, a laser may be directed onto the surface of the wafer and collected by a photodiode. The distance between the wafer and the light source may thus be determined. An optical focusing technique may not provide the most accurate focus or resolution, however, for tools (e.g., SEM or atomic force microscope (AFM)) other than optical microscopes.
Assuming that the tool is properly focused, the manufacturing process is monitored by performing line width measurements on a daily basis, to determine whether the CD SEM is reporting within the allowable statistical process control (SPC) limits for a given workpiece, such as a semiconductor wafer. Implicit in this method is the assumption that the tool has retained its focus. If, in fact, the tool is not at its optimal focus, and the line width measurements deviate from the baseline, the data are confounded, and it is not possible to accurately determine whether the process itself is operating within its normal specifications (and the tool resolution has changed) or the tool resolution is correct but the process is fabricating products that are out of specification.
For some SEM process and tool control methods, the SPC limits are extremely close to the limit of precision of current metrology techniques. New techniques are needed, therefore, to control CD in SEM metrology at increasingly more stringent levels. In addition, new techniques are required to accommodate new measurement tools, such at the AFM.